Backend and API development
A backend gives mobile and web products a reliable place for accounts, application data, permissions, business rules, notifications, administration, and integrations. FullStack Dev KZ builds FastAPI and SQL services as part of complete products or as focused backend work for an existing application.
A strong fit for
- Mobile products that need accounts, shared data, or administration
- Web and SaaS applications requiring secure business logic
- Companies connecting existing systems through a defined API
- Teams extending or replacing a fragile application backend
The backend is where product rules become enforceable
Interfaces can hide or disable controls, but the backend must decide what a user is allowed to do, validate data, apply calculations, manage ownership, and keep records consistent. It also creates the contract that lets mobile, web, administration, and external systems work with the same source of truth.
A useful API is designed around product behavior rather than database tables alone. Clear request and response models, authentication, permissions, errors, idempotency where required, and migration discipline make integrations easier to maintain and failures easier to understand.
Backend responsibilities we can deliver
The service can support a new product, extend an existing platform, or create a controlled bridge between systems.
Accounts and permissions
User registration, authentication, token handling, organisation boundaries, roles, and access to protected records.
Application data and rules
Typed models, validation, calculations, workflow states, history, and server-authoritative behavior.
APIs and synchronization
Interfaces used by mobile, web, administration, and approved external services, including resilient update flows.
Integrations and operations
Notifications, subscriptions, file workflows, third-party APIs, data import or export, deployment, and operational diagnostics.
Backend work starts with contracts and ownership
Before writing endpoints, we define the actors, data boundaries, business rules, and systems that will depend on the API.
- 01
Audit the current environment
Review existing clients, data, APIs, integrations, infrastructure, security assumptions, and known failure points.
- 02
Define domain and API contracts
Model records, states, permissions, validation, requests, responses, and error behavior around the product workflows.
- 03
Implement secure services
Build FastAPI routes, SQL models, migrations, authentication, integrations, and automated checks for important rules.
- 04
Connect real clients
Validate the API through the mobile or web journeys that consume it rather than testing endpoints only in isolation.
- 05
Deploy and hand over
Configure environments, secrets, migrations, logs, recovery expectations, and documentation needed for continued development.
Extend, integrate, or replace
An existing backend does not always need replacement. If its data model and authentication remain sound, focused endpoints, improved contracts, or an integration layer may solve the product need with less risk. Replacement is considered when the current service cannot safely support essential workflows or ownership boundaries.
Integration scope depends on the external system's API, authentication, rate limits, data quality, and operational responsibility. We avoid promising a connection before those constraints are visible. Where manual imports remain appropriate at low volume, they can be an intentional first phase rather than hidden technical debt.
Common API clients
- • React Native mobile applications
- • Next.js web applications
- • Administration and reporting tools
- • Approved third-party services
Useful review material
- • Current API documentation or code
- • Data model and integration list
- • Authentication and permission rules
- • Failure, migration, and deployment constraints
Related products and case studies
TradesMate backend
FastAPI, SQLAlchemy, PostgreSQL, authentication, business-scoped records, and subscription-aware access.
View this projectPubPlay live services
Shared venue, event, player, fixture, score, leaderboard, and display data across multiple clients.
View this projectIskra Trading API
A FastAPI service supporting catalogue, account, ordering, and business integration workflows.
View this projectRelated software decisions
These guides help define what should be built before selecting or scoping this development service.
Questions about backend & api development
Can you build a backend for an existing mobile app?
Yes. We review the current client behavior, data assumptions, authentication, and release constraints before defining a compatible API and migration path.
Can you integrate with an existing API?
Yes when the service exposes suitable, documented access. We assess authentication, contracts, limits, errors, and data ownership before estimating integration work.
Can one API support both web and mobile apps?
Yes. Shared domain rules and data can serve multiple clients while responses and workflows remain appropriate to each interface.
Can you replace or extend an existing backend?
Both are possible. Extending is often lower risk when the current foundation is sound. Replacement requires a clear migration, compatibility, data, and deployment plan.
What helps us scope your project
A complete technical specification is not required. A clear description of the workflow and constraints is enough to start shaping the right next step.
Read the software estimation guide- The people who will use the software and the main workflow they need to complete
- The platforms involved: mobile, web, administration, backend, or a connected combination
- Existing systems, APIs, spreadsheets, data, or manual processes that must remain or be replaced
- Required integrations, fixed deadlines, release constraints, and must-have outcomes
Make the system responsibilities explicit
Share the current clients, data, integrations, authentication model, business rules, and the backend problem that is blocking the product.